1. Field of the Invention
The invention relates to an optical component comprising a material which has a coefficient of thermal expansion α, with the coefficient of thermal expansion being dependent on location.
2. Description of the Related Art
In the present application the coefficient of thermal expansion α (CTE) is understood as the first derivation of the thermal expansion ΔL/L, the so-called thermal expansion, according to temperature.
Such optical components are of special interest in the field of X-ray lithography. This applies especially to lithography with soft X-rays, the so-called EUV lithography in the wavelength range of 10 to 30 nm. Mirrors with a high reflectivity in the X-ray range are used as optical components in the field of X-ray lithography. Such mirrors can be operated close to perpendicular incidence as so-called normal-incidence mirrors or in grazing incidence as so-called grazing-incidence mirrors. Mirrors are called grazing-incidence mirrors when the rays of a ray bundle impinging on the mirrors impinge under angles of β>70° relative to the surface normal.
X-ray mirrors with a high reflectivity in the X-ray region which are operated as normal-incidence mirrors comprise a substrate material and, based thereon, a multilayer system, e.g. a Mo/Si multilayer system or a Mo/Be multilayer system or a MoRu/Be multilayer system. With such systems it is possible to achieve in the EUV range reflectivities of over 50% and even of over 60%.
Depending on the wavelength of the light to be reflected it is also possible to use layer systems of other materials.
In addition to X-ray mirrors which are operated as normal-incidence mirrors, it is also possible to use mirrors operated in grazing incidence, i.e. so-called grazing-incidence mirrors. Such mirrors also comprise a substrate material. A simpler layer system is applied to the substrate material. The applied layer can be a ruthenium, palladium or rhodium layer.
It is desirable that high image-forming qualities are achieved for X-ray mirrors used in the field of EUV lithography, especially in projection objectives.
Since X-radiation (as explained above) is reflected completely neither under normal incidence nor under grazing incidence, energy is introduced into the mirrors, so that the mirrors or other optical components of an EUV system will heat up. The increase in temperature of the respective optical component by absorption of the EUV radiation impinging upon the optical component or mirror leads to the consequence that the image-forming quality is influenced as a result of thermal expansion. Other heat sources that can lead to the heating of the optical components such as the mirrors are the mirror actuators for example.
The minimization of thermal effects in optical components which are used in EUV projection systems is the subject matter of EP 0 955 565.
For the purpose of suppressing thermal effects, the mirrors known from EP 0 955 565 comprise a metallic substrate as the substrate material. Owing to the favorable thermal conductivity of the metals, the heat introduced into the mirrors is removed efficiently by a cooling apparatus preferably via the rear sides of the metal substrates. Although thermally induced mirror deformations can be reduced in this manner, it is not possible to avoid aberrations. Considerable aberrations still occur.